This invention relates to optical data storage media, and more particularly, to optical disks having low vertical birefringence and excellent pit/groove replication by the substrate (e.g., a transparent thermoplastic) which supports the reflective layer.
Optical data storage media, including optical disks as exemplified by compact audio disks and CD-ROM disks used in computers, have become a popular means of storing large amounts of digital data. The data stored on an optical disk are read by a plane polarized laser beam and a polarization-sensitive detection scheme. Specifically, in compact disk and DVD technology, the laser beam is typically reflected off of a reflective metallic surface which is supported by polycarbonate within a spinning disk. The metallic surface has pits which corresponding to binary data, which are arranged within concentric grooves. The optical disk is "read" by analyzing the reflected laser light to determine whether it impacted a pit. It is necessary to minimize polarization-dependent effects on these laser beams which may be caused by passage through the polycarbonate because such effects will distort the reflected laser light, thus preventing accurate reading.
Polycarbonates are the most commonly employed polymers in optical disks. Polycarbonates are particularly suited for this purpose because they are transparent and they have favorable physical properties.
In the further development of optical disks, particularly read-write disks and disks capable of storing larger amounts of data, various physical factors become important. One such factor, which is closely related to the storage capacity of the disk, is birefringence, i.e., the difference between indices of refraction for light polarized in perpendicular directions. Birefringence leads to phase retardation between different polarization components of the laser beam (i.e., a polarization-dependent effect), thereby reducing readability of the disk.
Birefringence has several sources, including the chemical nature of the raw material from which the disk is fabricated, the degree of molecular orientation therein and thermal stresses in a fabricated plastic optical disk. The observed birefringence of a disk is therefore determined by the molecular structure, which determines the intrinsic birefringence, and the processing conditions, which can create thermal stresses and orientation of the polymer chains. Specifically, the observed birefringence is typically a function of the intrinsic birefringence plus the birefringence introduced upon molding articles such as optical disks. The observed birefringence of an optical disk is typically quantified using a measurement termed "vertical" birefringence, which is described more fully below.
It is known that polycarbonates made from bisphenol-A (i.e., 2,2-bis(4-hydroxyphenyl)propane) have high intrinsic birefringence. It is also known that homopolycarbonates comprising units derived from spiro(bis)indanes, especially 6,6'-dihydroxy-3,3,3',3'-tetramethyl-1,1'-spiro(bis)indane (hereinafter "SBI"), have negative intrinsic birefringences, owing to the relatively rigid molecular structure of the SBI unit and its conformation in said homocarbonates.
A class of copolycarbonates having low intrinsic birefringence is disclosed, for example, in U.S. Pat. No. 4,950,731. Said copolycarbonates comprise structural units derived from bisphenol A and SBI.
It is also known, however, that SBI polycarbonates are deficient in such areas as processability and ductility. One result is that molding of SBI polycarbonates, including both homopolycarbonates and copolycarbonates also containing bisphenol A units, induces severe stresses. This is particularly true of injection molding of optical disks, in which such stresses are magnified. Under such conditions, these stresses can cause significant observed birefringence in a disk, despite the low intrinsic birefringence of the SBI-containing polycarbonates.
As mentioned previously, the observed birefringence is typically quantified for an optical disk by measuring the "vertical" birefringence (hereinafter "VBR"), which is defined as the difference between the refractive indices for light polarized perpendicular to the plane and that polarized in the plane of the disk. High VBR is a problem often encountered in disks molded from SBI polycarbonates. SBI polycarbonates may also have unacceptably high glass transition temperatures above 200.degree. C., and unacceptably high melt viscosities. VBR must be kept below a certain threshold to ensure proper reading of an optical storage media.
Japanese Kokai 4/345,616 discloses polyestercarbonates containing carbonate units derived from spiro(bis)indanes such as SBI and from bisphenols such as bisphenol A, and ester units derived from dicarboxylic acids such as sebacic acid and dodecanedioic acid (hereinafter "DDDA"). However, this patent does not discuss which types of such copolymers can be used to make an optical disk having acceptable pit/groove replication and low VBR.
It has been found, that the VBR's of disks molded from these copolyestercarbonates vary over a wide range, including VBR levels that are too high to allow accurate data reading. Moreover, the melt viscosities and glass transition temperatures of such copolyestercarbonates are often too high for complete mold filling when making optical disks. Also, the pits in the metallic surface may not be properly filled with the copolyestercarbonate. Any of these problems will prevent the manufacture of readable optical disks.
Optical disk grade polycarbonates can be prepared by conventional interfacial and melt polymerization methods as well as by redistribution as described, for example, in U.S. Pat. No. 5,414,057 and solid state polymerization.
It is of interest, in view of the above-mentioned deficiencies in prior art materials, to develop new methods of making optical disks having low VBR and excellent pit/groove replication.